Entanglement Between Distant Macroscopic Objects Could Improve LIGO Gravity Measurements

Researchers have demonstrated entanglement between distant objects (millimeters in size and with billions atoms in a magnetic field) in a hybrid system consisting of a mechanical oscillator and an atomic spin ensemble. This constitutes a new milestone for hybrid macroscopic entanglement and for demonstration of noiseless trajectories in the negative-mass reference frame. Future applications of the noiseless measurement of motion under realistic thermal conditions enabled by this work include, e.g., off-resonant continuous force detection and resonant pulsed measurements based on state preparation and retrodiction.

Niels Bohr Institute in Copenhagen used a 13 nanometre-thick, millimeters-long silicon nitride membrane (or drum).

Entanglement has previously only been demonstrated with very few atoms or with bose einstein condensates. Condensates are at extremely low temperatures.

How This Would Improve Gravity Detectors

This work could be used to make certain ultra-precise tools more sensitive. If the mirrors in the Laser Interferometer Gravitational-wave Observatory (LIGO) could be entangled then there would be less uncertainty. If LIGO is improved we could observe more distant collisions of black holes and neutron stars and we could understand more about gravity and the universe.

We could have instruments with quantum-mechanics-free measurement. It would be a step towards limitless precision of measurements of motion.

Gravity Wave detections are rapidly increasing. The first gravity wave was detected in 2015. They needed to create single wavelength lasers and use one-megawatt of power to detect differences of one trillionth of a wavelength. A white paper describes the research and development that will be needed over the next decade to realize “Cosmic Explorer”. This will be a detection system for blackholes across the Universe. A Cosmic Explorer with quantum mechanics free measurement would be even more useful and powerful. We are already on a path to go from about 100 gravitational wave detections per year to millions.

Cosmic Explorer together with a network of planned and proposed observatories spanning the gravitational-wave spectrum, including LISA and the Einstein Telescope will be able to determine the nature of the densest matter in the universe; reveal the universe’s binary black hole population throughout cosmic time; provide an independent probe of the history of the expanding universe; explore warped spacetime with unprecedented fidelity; and expand our knowledge of how massive stars live, die, and create the matter we see today.

Entanglement of Macroscopic Objects Towards Quantum Mechanics Free Measurement

Future entanglement work is described in a complete Arxiv paper. They are enhancing entanglement and achieving practical detection of noiseless trajectories of motion will primarily concentrate on amending experimental imperfections. This includes reduction of broadband spin noise by better mode-matching of light to the atomic ensemble, reducing optical losses and the cavity noise that appears from the mirror-modes, as well as motional drifts due to thermal fluctuations of the cryogenic chamber. Near term improvements are reducing optical losses by 10% and tripling readout levels and reducing noise to one third.

Nature Physics – Entanglement between distant macroscopic mechanical and spin systems


Entanglement is an essential property of multipartite quantum systems, characterized by the inseparability of quantum states of objects regardless of their spatial separation. Generation of entanglement between increasingly macroscopic and disparate systems is an ongoing effort in quantum science, as it enables hybrid quantum networks, quantum-enhanced sensing and probing of the fundamental limits of quantum theory. The disparity of hybrid systems and the vulnerability of quantum correlations have thus far hampered the generation of macroscopic hybrid entanglement. Here, we generate an entangled state between the motion of a macroscopic mechanical oscillator and a collective atomic spin oscillator, as witnessed by an Einstein–Podolsky–Rosen variance below the separability limit, 0.83 ± 0.02